Transparent polyamide and multi-ply barrier films containing same

ABSTRACT

The invention relates to a barrier layer resisting the transmission of oxygen for multi-layer barrier film characterized in that it is made of a transparent copolyamide comprising three components. The first component is hexamethylenediamine and adipic acid. The second component is hexamethylenediamine and azelaic acid, hexamethylenediamine and sebacic acid, and mixtures thereof. The third component is hexamethylene-diamine and isophthalic acid, hexamethylenediamine and terephthalic acid, and mixtures thereof. The total of all components is 100 mol %, and the ratio of the barrier effects in the barrier layer with respect to carbon dioxide and oxygen is at least 3:1.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to German application No. P 43 09534.8 filed Mar. 24, 1993, the entire specification of which isincorporated by reference.

1. Field of the Invention

The present application provides a transparent copolyamide, which may beused as a barrier layer in multi-ply barrier films, particularly forcheese packaging.

2. Technology Review

Multi-ply films containing polyvinylidene compounds are currently usedin the packaging industry. Examples that might be cited are described inEuropean Patent 368,808, U.S. Pat. No. 4,424,243, and U.S. Pat. No.4,801,486. The main drawback of polyvinylidene chloride, in addition tobeing difficult to process and having a strong corroding effect onprocessing machines, is the environmental impact its disposal has due tobe the release of chlorine and the possible formation of dioxins.

According to EP 457,598, the use of copolyamide 6/12 is also known. Onegreat disadvantage of this copolyamide material is its oxygen barrierproperty, which is affected by environmental humidity and decreases asthe humidity increases. EP 0 409 666, based on the U.S. application Ser.No. 07/383,135 filed Jul. 21, 1989, describes copolyamide compositionspolymerized from isophthalic acid, adipinic acid, alternativelyterephthalic acid, hexamethylenediamine and m-xylylene diamine havinggood gas barrier properties. Nevertheless this specification does notshow any ratio higher than 3:1 in the barrier resistance against CO₂ andO₂, nor how to minimize the effect of humidity on these resistances.

Blends having three or more polyamides, including preferably, partlyaromatic polyamides, allow relatively good barrier properties to beobtained. The disadvantages of the blends of three or more components,by comparison with copolyamides, are the problems with granulation, thegreat expense for testing each separate component, and the additionalextrusion step, which increases production cost considerably, adding tothe significantly higher cost for raw materials.

Moreover, the field of food packaging is in need of a barrier film thatdemonstrates good processing properties, does not have any corrosiveproperties, is halogen free, and which has barrier properties which donot deteriorate if humidity is absorbed.

SUMMARY OF THE INVENTION

The invention provides a transparent copolyamide comprising threecomponents. The first component is hexamethylenediamine and adipic acid(hexanedioic acid). The second component is hexamethylenediamine andazelaic acid (nonanedioic acid), hexamethylenediamine and sebacic acid(decanedioic acid), and mixtures thereof. The third component ishexamethylene-diamine and isophthalic acid (1,3-benzenedicarboxylicacid), hexamethylenediamine and terephthalic acid(1,4-benzenedicarboxylic acid), and mixtures thereof.

It is therefore an object of this invention to overcome each of theabove-mentioned disadvantages. A particular object of the invention isto provide a transparent copolyamide having special barrier behavior asis required for barrier films used in the field of food packaging,especially cheese packaging.

An advantage of the invention is to provide barrier films having a ratioof carbon dioxide barrier to oxygen barrier properties in the range of3:1 to 5:1 or higher.

DETAILED DESCRIPTION OF THE INVENTION

A transparent copolymer suitable to prepare a barrier layer, andmulti-ply barrier films including such a barrier layer according to theinvention are provided by the copolyamide films of the presentinvention. These copolyamide films are characterized in an advantageousmanner by good biaxial stretchability, high shrinkage of the stretchedmaterial, a high degree of transparency, sufficient flexibility andcompatibility. These copolyamide films may be produced, withoutdifficulty, by coextrusion in combination with other layers ofthermoplastically processable polymers, for example, supporting layers,sealing layers or protective layers, and may be used as multi-plybarrier films.

The transparent copolymer according to the present invention is atransparent copolyamide containing the following components:

I Hexamethylenediamine and adipic acid present from about 15 to 75 mol%;

IIa Hexamethylenediamine and azelaic acid and/or;

IIb Hexamethylenediamine and sebacic acid together present from about 15to 65 mol %; and

IIIa Hexamethylenediamine and isophthalic acid and/or

IIIb Hexamethylenediamine and terephthalic acid together present fromabout 10 to 70 mol %, with the total quantity being 100 mol %, ofcourse.

A preferred composition according to the present invention comprises

about 15 to 60 mol % of I;

about 15 to 55 mol % of IIa and/or IIb; and

about 10 to 45 mol % of IIIa and/or IIIb.

A particularly preferred composition according to the present inventioncomprises

about 33 to 55 mol % of I;

about 15 to 55 mol % of IIa or IIb; and

about 10 to 30 mol % of IIIa or IIIb.

The components of each copolyamide composition must total 100 mol %,respectively. It is apparent that the total acid and diamine must bepresent at a nearly equimolar ratio.

The copolyamides according to the invention may contain, among others,common additives such as chain regulators, stabilizers, anti-foamingagents, and crystallization accelerators.

The production of copolyamides according to the invention isaccomplished using the melt-condensation method in a known manner. Theproduction of barrier layers, in a multi-ply barrier film, for example,for testing and examination purposes, may be accomplished by means offilm-blowing extrusion in a known manner.

Depending on intended use of the copolyamide, the preferred range ofrelative viscosity of the copolyamides according to the invention(measured as a 0.5% solution in m-cresol at 23° C.) lies between about1.75 and 2.5. However, either higher or lower degrees of viscosity mayalso be used.

Also depending on the intended use, other barrier layers and a pluralityof further layers may be used together with a copolyamide layeraccording to the present invention for a multi-ply product. Otherpreferred polymer layers are protective and supporting layers which givedefined functional characteristics to the multi-ply product. Knownsealing layers may be used in order to render the multi-layer compoundsealable by means of hot sealing. Known layers of adhesion promotors mayalso be used advantageously. Preferred polymers for protective andsupporting layers are those made of polyamides, polyolefins orpolyesters. Advantageous adhesion promotor layers are functionalizedpolyolefins, i.e., they are provided with functional groups. Sealinglayers consists, in an advantageous manner, of low-melting polymers.So-called ionomers are preferred. The production of multi-ply filmproducts is advantageously accomplished with coextrusion equipment.

The following Examples elucidate the invention for the benefit of thoseof ordinary skill in the art, without limiting the broad scope of theinvention.

                  TABLE 1                                                         ______________________________________                                        The composition of the barrier layers in Examples 1-3.                               Composition of Copolyamides in Barrier Layers                          Example  I        IIa        IIIA   IIIB                                      No.      Mol %    Mol %      Mol %  Mol %                                     ______________________________________                                        1        38       52         10     --                                        2        55       15         30     --                                        3        38       52         --     10                                        ______________________________________                                         IIIA hexamethlyenediamine and isophthalic acid;                               IIIB hexamethylenediamine and terephthalic acid.                         

                  TABLE 2                                                         ______________________________________                                        Barrier behavior at 0 and 85% relative humidity                                            Barrier Behavior*                                                Example   Environ.            Carbon                                          No.       Humidity Oxygen**   dioxide**                                                                            Ratio***                                 ______________________________________                                        1          0% rH   65         204    3.1                                                85% rH   40         469    11.7                                     2          0% rH   48         144    3.0                                                85% rH   24         209    8.7                                      3          0% rH   75         228    3.0                                                85% rH   45         363    8.1                                      Comparative                                                                              0% rH   120        400    3.3                                      Example   85% rH   200        800    4.0                                      ______________________________________                                        *Blown film thickness 50 μm                                                **cm.sup.3 /m.sup.2 day bar                                                   Measuring devices:                                                             85% rH Mocon Oxtran Twin                                                       0% rH Lissy L 100                                                           ***Ratio of carbon dioxide to oxygen barrier                                    (rH: relative humidity)                                                 

COMPARATIVE EXAMPLE

Copolyamide CF 6S (Ems-Chemie AG, Zurich) a copolyamide on a caprolactamand laurinlactam base.

The barrier layers according to the invention in Example 1-3 show thesignificantly improved oxygen barrier with respect to the comparativeexample and the essentially better ratio of carbon dioxide to oxygenbarrier.

In a field test, in which packaged cheese was stored in a coolingchamber, the film material containing the copolyamide from Example No. 1proved to be successful. After six months' storage, no mildew formed andno swelling occurred due to the carbon dioxide formed during ripening ofcheese.

Table 3 is a comparison of a commercially used cheese package havingpolyvinylidene chloride as barrier layer and a multi-ply barrier film,according to the invention, containing the copolyamide from Example No.1 as barrier layer in combination with an inner ionomer sealing layer,an outer protective layer of polyolefin, and two adhesive layerssurrounding the barrier layer.

                  TABLE 3                                                         ______________________________________                                        Multi-layer packaging films:                                                  0% rH/85% rH                                                                  Barrier Layer  Barrier Behavior                                                       Thickness            Carbon-                                          Type    μ       Oxygen**  dioxide**                                                                              Ratio***                                ______________________________________                                        PVDC    10         160/175   800/800   5/4.6                                  Example 1                                                                             10         208/207   1323/2610                                                                              6.4/12.6                                ______________________________________                                        **cm.sup.3 /m.sup.2 day bar                                                   ***Ratio of carbon dioxide to oxygen barrier.                                 Measuring devices:                                                              85% rH                                                                              Mocon Oxtran Twin                                                       0% rH Lissy L 100                                                       

The multi-ply barrier film having a copolyamide barrier layer accordingto the invention makes possible a sufficient oxygen barrier as well asan excellent barrier ratio of carbon dioxide to oxygen.

A special advantage of copolyamides as a barrier layer compared tobarrier films containing polyvinylidene chloride is the independence ofthe oxygen barrier from environmental humidity. Further advantages aregood flexibility, the lack of corrosive properties and, above all,freedom from the release of halogens.

The oxygen barrier of the copolyamide barrier layers according to theinvention may be further improved in the dry state by adding knownnucleation substances, for example, polyamide 22 or mineral nucleationsubstances such as talc. The commonly known nucleation substances areused in commonly known concentrations. Moreover, due to this measure, astable barrier level is more rapidly obtained after film extrusion.

It is understood that various other modifications will be apparent toand can readily be made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A transparent polyamide, consisting of:(a) afirst component comprising hexamethylenediamine/adipic acid and presentfrom about 15 to 75 mol %; (b) a second component selected from thegroup consisting of hexamethylenediamine/azelaic acid,hexamethylene/sebacic acid, and mixtures thereof and present from about15 to 65 mol %; and (c) a third component selected from the groupconsisting of hexamethylenediamine/isophthalic acid,hexamethylenediamine/terephthalic acid, and mixtures thereof, andpresent from about 10 to 70 mol %, said first, second and thirdcomponents totaling 100 mol %, and said polyamide exhibiting barriervalues having a ratio of at least 3:1 with respect to carbon dioxide andoxygen.
 2. The transparent polyamide according to claim 1, wherein saidcopolyamide consists of:(a) said first component comprisinghexamethylenediamine/adipic acid and present from about 15 to 60 mol %;(b) second component selected from the group consisting ofhexamethylenediamine/azelaic acid, hexamethylenediamine/sebacic acid,and mixtures thereof, and present from 15 to 55 mol %; and (c) saidthird component selected from the group consisting ofhexamethylenediamine/isophthalic acid, hexamethylenediamine/terephthalicacid, and mixtures thereof, and present from about 10 to 45 mol %. 3.The transparent polyamide according to claim 2, wherein said polyamideconsists of:said first component present from about 35 to 55 mol %; saidsecond component present from about 15 to 55 mol %; and said thirdcomponent present from about 10 to 30 mol %.
 4. The transparentpolyamide according to claim 1, wherein said polyamide has a relativeviscosity between about 1.75 and 2.50 measured as 0.5% solution inm-cresol at 23° C.